Masked men in camouflage toting Kalashnikov rifles fan out through a dusty olive orchard, part of a group of Hezbollah-backed fighters from Lebanon who are patrolling both sides of a porous border stretch with Syria on April 12, 2013. (Bilal Hussein/AP)

BEIRUT — Lebanese groups on opposite ends of the nation’s polarized political spectrum are starting to play a more public role in the Syrian civil war, rendering Lebanon’s stated policy of neutrality toward the two-year conflict increasingly obsolete and threatening the tenuous stability of this Arab country.

Although the Lebanese militant and political group Hezbollah has acknowledged little about its role in the fighting next door, Syrian rebels and an analyst close to the Shiite organization, a longtime ally of Syrian President Bashar al-Assad, say it has amplified its operations inside Syria in recent weeks, adding muscle and firepower to an ongoing Syrian military offensive to retake a strategically important border zone from the rebels.

That acceleration has provoked a backlash from Hezbollah’s Sunni opponents inside Lebanon, who had mostly stayed on the sidelines until now. This past week, two prominent firebrand clerics publicly called on their Sunni followers to go to Syria to fight Hezbollah on behalf of Syria’s predominantly Sunni rebels. One of the clerics, Ahmad ­al-Assir, said that his call to jihad had already galvanized a volunteer force of hundreds of young men.

Syria’s increasingly sectarian civil war has yet to spill deeply into religiously diverse Lebanon, where more than 60 percent of the population is Muslim, split roughly between Sunnis and Shiites. But political analysts say opposing Lebanese groups’ growing involvement in the conflict next door is exacerbating dangerous tensions that have periodically erupted into sectarian clashes in the past year. In recent weeks, Syrian rebels have intentionally fired shells at Hermel, a Lebanese border town near al-Quseir that is a Hezbollah stronghold.

Echoing a view that is gaining ground among Lebanese, Assir and Mohammad Obeid, a Shiite analyst with close ties to Hezbollah, said that Lebanon’s official policy of neutrality toward the Syrian war is no longer worth taking seriously.

Interactive Grid: Keeping track of the conflict in Syria through videos, images and tweets.

“Everyone has violated it. So it’s not only Hezbollah,” Obeid said. “Everyone is now acting on a sectarian basis toward the Syrian crisis.”

The increasingly overt public polarization in Lebanon over Syria’s war — and an expanding debate in the news media and mosques over Hezbollah’s participation in it — could expose Lebanon to a regional backlash if Syrian rebels and the Arab Gulf states backing them decide to target Hezbollah on its own turf, said Paul Salem, director of the Carnegie Middle East Center in Beirut.

Hezbollah’s movement in the Syrian border region of al-Quseir indicates “a new type of engagement, a new threshold” that, coupled with the Sunni reaction it has provoked, is “one more step in a bad trajectory,” Salem said.

Increased activity, fighting

Political analysts and Syrian opposition activists said Hezbollah has been involved in the Syrian conflict to some extent for more than a year, despite the group’s public claims that its members are merely protecting Shiites on the Lebanese side of the border. But recent reports from residents and rebels in the al-Quseir region suggest a new level of intensity.

Syrian opposition activists have posted online videos in recent days of alleged firefights with Hezbollah’s paramilitary groups and of Hezbollah identification papers allegedly found inside Syria. Hezbollah has also quietly held funerals for members purportedly slain in Syria’s fighting.

Assir, the extremist Sunni cleric, has accused Shiite officials in the teetering Lebanese government of bowing to Hezbollah, a powerful political force in Lebanon that the United States considers a terrorist organization.

“The Lebanese foreign ministry, the Lebanese government and the army are all working in favor of the Syrian regime,” he said in an interview in the southern Lebanese coastal town of Sidon, where he recently began recruiting jihadist fighters. “The neutral policy that the government claimed to have was not neutral. It was just turning a blind eye to Hezbollah involvement.”

In a video posted online Wednesday, Hadi al-Abdullah, a Syrian opposition activist using an alias, stood before a group of rebels in al-Quseir and displayed one Lebanese national identification and one Hezbollah party identification that bore the same name. Fighters in the video said they had found the documents after a shootout with the group.

“This is foolproof evidence that there are troops belonging to Hezbollah, the Lebanese party, inside al-Quseir, and they are killing Syrians,” Abdullah said.

In an interview conducted via Skype, Abdullah said that rebel intercepts of Hezbollah communications lines inside the border indicated that Hezbollah activity has progressed from helping to actively leading the Syrian regime’s three-week-old offensive in the area.

“They are giving orders to officers and troops of the regime army when to shell or to not to shell,” he said.

Obeid, who said he speaks daily with Hezbollah leaders and frequently travels to Damascus to meet with regime officials, corroborated that claim, saying the Syrian military is increasingly drawing on Hezbollah’s deep experience in unconventional warfare.

“Hezbollah is helping in putting the plans” in place, Obeid said. “The Syrian army is big and strong and has good leadership, but Hezbollah also has the experience in street fighting.”

The border zone of al-Quseir is a critical lifeline connecting Damascus and other major cities like Homs to Lebanon.

“The main plan is to open the road,” he said, referring to the major north-south highway that connects Damascus to Homs and the north and runs past al-Quseir. He said Hezbollah also plans to open routes northwest from Homs to regime strongholds along Syria’s coast.

“The towns and villages around this road were controlled by the fundamentalists,” he said, using a term that Syrian regime supporters frequently employ to describe opposition fighters. “So now [Hezbollah and regime forces] are liberating these villages and towns,” he said.

The Syrian regime’s survival is critical to Hezbollah’s existence. Syria has long been a powerful ally to the group and a conduit for Iranian money and weapons, which Hezbollah uses to support its overarching mission of “resistance” to Israel, Lebanon’s southern neighbor, and to maintain political muscle in Lebanese politics.

‘New enemy for Hezbollah’

“If the fundamentalists succeeded in removing the Syrian regime — and I do not believe they will — this would make [Syria] into a new enemy for Hezbollah,” Obeid said.

Without Assad, Hezbollah “would be besieged, surrounded in that area” by political foes, he said.

Those foes know it.

The uproar over Hezbollah’s involvement has rippled through Lebanon’s small community of extremist Sunnis, who are widely dismissed here as too weak to make an impact but who say they are ready to fight nonetheless.

Mohamed Kebrit, 36, is one of them. He has sympathized with the Syrian rebels since the conflict began, he said. But he did not decide to fight until Assir, a cleric whom he admires, urged Sunnis this week to do so.

Kebrit said he has only vague memories of his own country’s sectarian civil war, which raged in the 1970s and ’80s and cut such deep fault lines through Lebanese society that they underlie politics today. But Kebrit said the hostilities next door have given him a taste of the past — and of what might come.

“What Hezbollah is doing in Syria is creating hatred and animosity in Lebanon,” he said. “I had Shiite friends, and now they are defending what Hezbollah is doing,”

This week, the day after he signed up for jihad, he threw one of those friends out of his cellphone shop following a heated argument about Hezbollah in Syria.

“It’s creating tension,” Kebrit said. “You can touch it.”

Suzan Haidamous and Ahmed Ramadan contributed to this report.

Abigail Hauslohner covers transportation and development for The Washington Post. Previously, she served as the Post’s Cairo bureau chief.

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